refactor: extract ComboBox styling into dedicated module, add location step

- Create src/ui/combobox.rs with helper functions (show_combo_box, selectable_value, selectable_value_custom)
- Create src/ui/combobox_stil.rs with ComboBox visual constants (weak_bg_fill, text/border colors)
- Move theme.rs ComboBox visuals (weak_bg_fill, fg_stroke, bg_stroke) into combobox_stil::apply_visuals
- Move LocationStep from installer.rs to src/steps/location.rs
- Re-register LocationStep in steps/mod.rs
- Update all ComboBox usages across steps (bootloader, display_manager, partition, funct) to use theme::show_combo_box / selectable_value
- Replace manual mesh drawing in language selector with combobox helper
- Add region/timezone i18n keys
- Vertical-align region/zone labels to match ComboBox height

locales/en.toml

@@ -401,6 +401,7 @@ job_media_check            = "Verifying installation media"
 # ── CLI / Auto-install ─────────────────────────────────────
 hostname = "Hostname"
 timezone = "Timezone"
+region = "Region"
 username = "Username"
 
 # ── Slideshow (fallback) ───────────────────────────────────

locales/tr.toml

@@ -422,6 +422,7 @@ lvm_lv_action    = "İşlem"
 # ── CLI / Otomatik Kurulum ─────────────────────────────────
 hostname = "Bilgisayar adı"
 timezone = "Zaman dilimi"
+region = "Bölge"
 username = "Kullanıcı adı"
 
 # ── Slayt Gösterisi (yedek) ────────────────────────────────